We analyzed the available evidence on REGN1033 and muscle mass in mice, and what we’ve found so far suggests that daily injections of REGN1033 at 10 mg/kg are associated with increases in skeletal muscle size and muscle fiber cross-sectional area in both young and aged mice [1]. The evidence we’ve reviewed includes 16 studies or assertions that support this observation, with none that contradict it. Importantly, these changes occurred without altering the total number or type of muscle fibers, meaning the growth appears to come from existing fibers getting larger, not from new ones forming. This pattern was consistent across the studies we reviewed, and the effect was observed in two different age groups, which may suggest the response isn’t limited to young animals. However, we did not see data on how long the effects lasted, whether they reversed after stopping treatment, or if there were any side effects. We also don’t have information on whether these changes translate to improved strength or function in the mice. The evidence we’ve reviewed leans toward REGN1033 having a measurable impact on muscle size in mice under these specific conditions, but we can’t say whether this would apply to other doses, different strains of mice, or other species. In everyday terms: if you give mice this antibody every day at this dose, their muscles tend to get bigger — not because they make more fibers, but because the fibers they already have grow larger. But we don’t yet know if that means they become stronger, or what happens after treatment stops.